Degeneration of the cerebellum causes ataxia, a disorder of balance and coordination which can be severely disabling and render afflicted individuals unable to walk, speak, or make coordinated movements. Genetic ataxia affects half a million people worldwide, but current testing only identifies roughly 50% of the responsible genes. Better diagnosis will lead to better treatment and uncovering novel disease genes could provide new insight into the workings of the cerebellum and suggest ways to combat its degeneration. New technology, termed next-generation sequencing, is a cost-effective means of examining either the entire human genome or the key portion that codes for protein, known as the exome, and can be used to comprehensively investigate patients for novel genetic mutations. The UCLA Ataxia and Neurogenetics Program possesses a unique resource for studying this genetic variation with a large bank of DNA samples from hundreds of individuals and families with cerebellar ataxia, all with precisely detailed clinical information. 1) We will examine the exomes of 20 ataxic families with dominant (10) or recessive (10) inheritance, all screened negative for the most common known genetic ataxias. We will initially confirm probands possess no variants in rare (<1% worldwide) ataxia genes and then proceed to identifying novel causative variants. We have developed a pipeline for excluding benign polymorphisms and isolating only those variants with the highest probability of causing neurologic disease. For patients and families remaining unidentified following this analysis, we will examine their genomes for disease-causing variants potentially altering gene expression and/or regulation. We will estimate the occurrence of mutations in genes we discover within our ataxia patient population and confirm that they are not found in normal individuals. 2) Although examination of families is essential to the identification of new disease genes, the majority of undiagnosed patients seen in ataxia clinics lack a clearly defined family history of disease. To address these individuals, we will examine subgroups of patients with similar detailed clinical presentations as they will be most likely to share common genetic causes. We will initially apply our analysis pipeline across a specific well- defined subtype of pure cerebellar ataxia to isolate potential disease-causing genes and mutations. 3) Lastly, we will screen identified variants in a cell culture system using human neurons with a variety of tests designed to quantitate the severity of the mutations on basic cellular functioning. This information will be used to characterize and prioritize variants of unknown clinical significance for further detailed analysis. In summary, this study will provide a comprehensive view of the genetic etiologies behind cerebellar ataxia, define new molecular pathways important to the normal function of the cerebellum, contribute to advancements in clinical diagnosis, and help elucidate the mechanisms causing degenerative ataxia.